1. Field of the Invention
The present invention relates to an electromagnetic ultrasonic apparatus and, more particularly, to an electromagnetic ultrasonic apparatus which permits examining the quality of conductive materials, for example, metal strips, ingots and bullet type steel and aluminum without being brought into contact therewith.
2. Description of the Prior Art
An electromagnetic ultrasonic apparatus permits transmitting and receiving ultrasonic waves as it is in a non-contact state, and is useful as an instrument for examining materials, without destroying the same, for example, moving materials, and materials which are difficult to physically contact due to the temperature or shape thereof.
An electromagnetic ultrasonic apparatus comprising a transmitting magnetic pole provided such that the front end surface of the transmitting magnetic pole can be brought close and in opposition to one surface of a material to be examined, a receiving magnetic pole which is provided such that the front end surface of the receiving magnetic pole can be brought close and in opposition to the opposite surface of the material and which has a magnetic polarity opposite to that of the transmitting magnetic pole, an exciter coil for supplying a DC magnetic field to the transmitting and receiving magnetic poles, and transmitting and receiving coils provided at the front end surfaces of the transmitting and receiving magnetic poles is described in, for example, "British Journal of NDT", July 1977, pages 178-184, G. J. Parkinson and D. M. Wilson.
In the above apparatus, a mode conversion echo occurring when ultrasonic waves are reflected within a material being examined is received as noise and cause the SN ratio to be decreased. Therefore, this apparatus is not sufficiently high in capability of detecting flaws in a material.
Japanese Patent Laid-open No. 86388/1977 shows that, when two electromagnetic ultrasonic apparatuses are arranged close to each other to utilize the difference between output voltages occurring in both of the receiving coils, the mode conversion echo and external noise are deadened with each other. In this apparatus, the difference between waves passed through a material being examined and back face echo can be obtained as an output from the receiving coils so that the capability of this apparatus of detecting flaws in a material is improved to a considerable, but not sufficient, extent.